This invention relates to a heat-insulating closed structure featuring low cost, high air-tightness and heat-insulating property, light weight and great durability which has a composite membrane unit comprising an outer membrane formed of a gas-impervious substance, an inner membrane disposed at a distance from said outer membrane and formed with either a gas-impervious membrane having small holes punched therethrough at fixed intervals or a membrane of gas-pervious texture and a foamed heat-insulating member interposed to fill the space between said two membrane. The present invention relates further to a method for the manufacture of said heat-insulating closed structure.
With the advance achieved in recent years in techniques for the formation of foamed products, the method which enables structures to be built directly by foam molding has reached the point of being widely practiced. Consequently it has now become possible to build a structure by effecting foam molding at a desired building site. The foaming substance to be used in this foam molding can be selected from a rich variety of foaming compositions. Particularly in the case of urethane resins, foamed products of varying properties have become obtainable in consequence of the progress attained in research and development. Moreover, urethane resins enjoy the advantage of being moldable under a wide range of foaming conditions.
One shortcoming of the conventional method for foam molding large structures at building sites has been its susceptibility to weather conditions. In addition, it has been pointed out that the techniques conventionally employed in foam molding entail various problems such as are described herein below.
As the means for forming foamed products, there have heretofore been adopted the mold process which uses no molding dies and the spray process which uses no molding dies.
In effecting foam molding by the mold process, there is required a retaining device strong enough to withstand the pressure of foaming. This retaining device becomes voluminous and expensive when the foamed products have particularly large dimensions.
As a typical example of this kind of foam molding heretofore practiced, there may be cited a process which comprises erecting a form conforming to a desired structure. by suitably assembling panels made of wood, metal or synthetic resin, casting a foaming dope into the spacer enclosed by said panels, thereafter causing said dope to foam and allowing the foam to set.
A heat-insulating structure which uses heat-insulating panels having foamed urethane slabs coated with metal sheets has already been adopted for use in combination freezer-refrigerators. A constantly recurring complaint about such refrigerators is, however, that they are expensive and that they fail to provide desired heat-insulation at areas where such component panels are joined. An attempt to manufacture larger panels with a view to decreasing such joint areas inevitably requires a marked addition to the dimensions of the production facilities and consequently boosts the cost of equipment. In this case, there ensue other problems such as, for example, the selection of the position for an inlet through which the foaming dope is introduced into the cavity of the form, the displacement of air with the foaming dope particularly in a form of complicated design, and the difficulty experienced in ensuring uniform distribution of the foaming dope throughout the cavity of the form.
While the technical difficulties described above are encountered, the major disadvantages suffered in effecting the process in actuality are as follows:
(1). Lack of uniform distribution of foaming dope. PA1 (2 ). Occurence of large voids due to residue of air bubbles. PA1 (3). Unpredictable deformation due to secondary foaming.
When a large structure is built by use of urethane foam, voids occur in the structure interior and these voids may possibly lead to surface decays and to structure deformation, permitting development of dangerous and serious cracks. This phenomenon is frequently observed. These voids and the cracks which develop from such voids constitute the most fatal flaw for foamed products.
These voids may be ascribed to various causes. For example, since the form conforming to the desired structure is obtained by assembling component panels, there is a possibility that portions susceptible to stagnation of air may occur within the cavity enclosed with the panels. It is also possible that frictional force is generated between the inner wall surface of the form and the adhering bubbles of the foam while the foaming dope is being poured into the form or it is undergoing solidification, with the result that the formed bubbles are deformed or merge to give rise to large voids.
To preclude such problems, it is necessary that due consideration be paid to heightening the degree of surface finish of the inner walls of the form and also giving the form a shape containing the fewest possible corners. In addition, various improvements have been tried with respect to the shape and dimensions of nozzles used for the introduction of foaming dope. It is, however, difficult to fix a definite set of standards in this respect. The object of preventing formation of voids may be attained to some extent by, for example, providing the form with a multiplicity of inlets for introducing the foaming dope into the form cavity. This work, however, requires much labor.
The spray process developed for foam molding is an excellent way of manufacturing foamed products on the spot. It enables a structure of any desired shape to be built or produced with a foamed substance at the site of building by using a foaming dope of rigid urethane. According to this process, however, the urethane resin is apt fly around the form to degrade the yield of work and, what is more, defile the environment. The foamed product has an extremely rough surface and, therefore, entails much labor for surface finishing. When such work is performed outdooors, the surface of the foamed product which is sprayed must subsequently be coated so as to avoid possible effects of weather and particularly to prevent penetration of humidity. For this purpose, the work of covering the foamed product with a sheet of material impervious to water and humidity or coating the foamed product with a similar protective film is required to follow the step of spraying. It is not an easy job to drape a large structure on all of its outer faces with a covering material especially when the work must be done outdoors. This work necessitates use of large scaffoldings and a hoisting machine and requires highly-skilled labor. For a sheet-shaped cover to provide required protection for the foamed product from humidity, the edges of said cover must be handled with particular care lest they should develop leaks to water or humidity. Perfect protection of the sprayed foamed product from water or humidity cannot be obtained by simply having said product draped with a sheet material. The ends of such sheet material placed to drape the foamed product are loose enough to admit rain water or other similar liquid. It is well known that prevention of invasion by humidity is particularly difficult. Once humidity begins to find access to the inside of the cover, it converts itself into vapor and the vapor diffuses into the individual cells and fine crevices in the foamed product. When this foamed product is used in a freezer or freezing facility, the vapor trapped therein is condensed and frozen to the extent of having an adverse effect upon the thermal behavior of the gas cells. The freezing of the condensed vapor can, in the worst case, fracture the individual cells in the foamed product.
A principal object of this invention is to provide a heat-insulating closed structure which is simple to build, permits necesssary handling outdoors as well as indoors and excels in heat-insulating property.
Another object of the present invention is to provide a method for the manufacture of a heat-insulating closed structure which is simple to build, permits necessary handling outdoors as well as indoors and excels in heat-insulating property.